ing some of his measurements for the geometric acceptance of his apparatus, he was told to ask for help from a bright young theoretical student from New York. According to Heinz, his fellow student looked at Heinz's carefully defined problem in a rather disinterested manner and mumbled something to the effect that he would look at it if he had a chance. A few days later the student, Richard Feynman, presented Heinz with a carefully written, elegant solution to his problem.

In 1939, a little more than a year after Barschall had begun work at Princeton, Niels Bohr arrived with the news of the discovery of fission and urged Ladenburg and his colleagues to conduct some experiments on the new fission process using fast neutrons. Kanner and Barschall had been working with 2.5-MeV neutrons produced through the D + D ? ³He + n reaction by deuterons accelerated to 400 keV by a transformer-rectifier set striking a D²O ice target. Interrupting his thesis research, in a few days work with Ladenburg, Kanner, and Van Voorhis, Barschall demonstrated the fission of uranium from the interaction of 2.5-MeV neutrons and measured the cross-section and energy yield. While slow neutron fission proceeded through interactions with the recently discovered rare isotope ²³⁵U, they demonstrated that the fast neutron fission was dominated by neutron interactions with the primary isotope ²³⁸U.

Half a year later after further measurements of fast neutron fission, Heinz resumed his thesis work with Kanner on the scattering of 2.5-MeV neutrons by the lightest nuclei. In 1940 John Wheeler and Barschall showed that the measurements Heinz made with Kanner of the angular distribution of neutrons scattered from helium showed conclusively that spin-orbit nuclear forces were very strong—much stronger than anyone had then believed. Both the experimental results and the analysis can now be seen to be valid and con-